John Randall, head of the MRC Biophysics Unit at King's College, London writes to Rosalind Franklin in Paris, stating that Maurice Wilkins and Alec Stokes intend to cease working on DNA, and that her planned research should therefore switch from protein structure to DNA structure. Randall says: "This means that as far as the experimental X-ray effort is concerned, there will be at the moment only yourself and Gosling...". Crucially, the letter is not seen by Maurice Wilkins, Ray Gosling's PhD supervisor, and the Unit's leading researcher on DNA structure. In fact, Wilkins does not become aware of what he has called Randall's 'secret letter' until many years later -- after Franklin's death.
While Wilkins is away on holiday in Wales, Rosalind Franklin arrives at King's. She and Ray Gosling begin work on calf thymus DNA samples given to Wilkins by Rudolf Singer in May 1950.
Tuesday-Friday 22-25 May
James Watson attends a conference at the Naples Zoological Station, where Wilkins shows a relatively poor X-ray diffraction photograph of the A form of DNA that he and Gosling have taken. This, however, ignites Watson's enthusiasm; if DNA can be crystallised its structure must be regular and therefore open to straightforward interpretation. Watson attempts to speak to Wilkins on an excursion to the Greek temples at Paestum, but Wilkins avoids him. (Wilkins later claims that he does not understand Watson's talk of bacteriophages.)
Maurice Wilkins travels to the USA to attend the Gordon Conference in New England in place of Randall. There, Erwin Chargaff gives him DNA specimens from E. coli, wheatgerm and pig. [This ultimately proves to be of inferior quality and does not give the regular X-ray diffraction patterns of Singer's material observed by Franklin.]
Watson arrives in Cambridge from Copenhagen to work at the Cavendish Laboratory and finds himself sharing an office with Francis Crick and Robert Parish. In theory (to satisfy his financial sponsors) he's supposed to be working on the structure of proteins and plant viruses whereas in reality he finds himself (unsuccessfully) attempting to crystallise myoglobin for John Kendrew's X-ray diffraction studies.
Thursday 1 November
Inspired by the work of Vladimir Vand, Francis Crick and Bill Cochran work out how to interpret X-ray diffraction photographs of helical molecules (i.e., the necessary Fourier Transform). This is published the following year in Acta Crystallographica. The authors acknowledge that the same theory was also arrived at independently by Wilkins's colleague Stokes at King's College. (Stokes modestly declines to be cited directly in the paper, however.)
Wednesday 21 November
Watson attends a colloquium on DNA at King's College, London. There, Stokes, Wilkins and Franklin speak, but it is Franklin's X-ray diffraction slides of DNA crystals that capture Watson's attention. [Franklin's notes suggest that she showed pictures of varying quality of both the 'wet' B form and the 'dry' A form of DNA fibres; the only other account, Watson's, is short of details. The recognition of the two forms is a crucial breakthrough.] Watson does not take notes at the meeting and since he has only recently started to learn about X-ray crystallography the details he reports to Crick on his return to Cambridge are misleading. In particular, he fails to appreciate that the high water content of the molecule means that the polar groups (the phosphates) must be on the outside.
Monday 26 November
Watson and Crick rapidly build a triple helix model of DNA, based on the little Watson can remember from the King's colloquium. It's their first ever model; and it's wrong in several respects (e.g., it has the phosphates on the inside and is held together by magnesium ions). At the time they are quite pleased with it, however.
Tuesday 27 November
Kendrew insists that the three-stranded model is shown to the King's scientists as a matter of courtesy, so Crick telephones Wilkins. The next day Wilkins, his collaborator Bill Seeds, Bruce Fraser, Franklin and Gosling travel to Cambridge to see the flawed construction. It is clear within minutes that the model is wrong [Ray Gosling said that Franklin laughed at the model] and the London group heads home on the afternoon train.
Wilkins writes to Crick, asking him and Watson to stop work on DNA structure. The Director of the Cavendish, Bragg, comes to an agreement with John Randall that Watson should be confined to working on his official subject, plant viruses (TMV), while Crick should restrict himself to his PhD on polypeptides and proteins. The metal parts used to make the ill-fated model are sent to the London group, but they are never used.
Wednesday-Thursday 16-17 April
Watson attends a Society for General Microbiology conference in Oxford in place of his mentor Salvador Luria, who has had his passport withdrawn by the US authorities during the McCarthy-inspired 'witchhunt'. Watson has just received a letter describing as yet unpublished work by Al Hershey and Martha Chase which he reports to the meeting. Their 'Waring Blender' experiment with radio-labelled phage provides conclusive evidence that DNA is the genetic material. Watson is convinced, although most delegates remain sceptical about the interpretation of Hershey and Chase's (possibly) inadequately-controlled work.
Thursday 1 May
Franklin shows her early X-ray diffraction photographs of DNA to Robert Corey after a meeting at The Royal Society. (Corey attends the meeting in place of Linus Pauling, who has had his passport confiscated while waiting to board a plane in the USA). Corey is very impressed with the photographs and thereafter maintains a correspondence with Franklin, although he probably doesn't pass any vital data to Pauling. [Wilkins had previously declined to send copies of his X-ray photographs to Pauling.]
Friday 2 May
'Photograph 51' of the B form of DNA is taken (the long exposure was actually started the previous day). (In 2002, Watson suggested that the photo was probably taken by Franklin's PhD student, Ray Gosling.)
Tuesday 1 July (approximately)
As a result of a deteriorating relationship with Wilkins, Franklin arranges to transfer from King's to Birkbeck College, London for the final year of her three-year fellowship.
Friday 18 July
Franklin (known as 'Rosy' behind her back) pens a humorous obituary for the helical interpretation of the A form of DNA. This joke reportedly upsets Stokes and Wilkins (known as 'Uncle Maurice' behind his back).
End of September
Peter Pauling (son of Linus Pauling) and Jerry Donohue (a former student of Linus Pauling) arrive at the Cavendish and share an office with Watson and Crick.
Monday 15 December
An MRC committee visits King's to see the work of the Biophysics group. They receive the written report which gives details of Franklin's X-ray diffraction studies of the B form of DNA.
Wednesday 17 December (approximately)
Peter Pauling receives a letter from his father saying that he and Corey have a structure for DNA, but no details are given. Peter Pauling tells Watson, who passes the news to Wilkins as he passes through London en route to a winter skiing trip.
Wednesday December 31
Pauling and Corey post their manuscript with their proposed structure of DNA to the Proceedings of the National Academy of Sciences. On the same day Pauling writes to his son asking whether he would like a copy of it.
Thursday 1 January
Franklin is supposed to leave King's College for Birkbeck, but her departure is delayed by a month by a bout of 'flu.
Friday 2 January
Pauling and Corey send a brief note to Nature announcing their structure and saying that details will appear in the February issue of the Proceedings.
Tuesday 6 January
Franklin writes to Corey, asking for a copy of his and Pauling's paper. [Corey does not reply until 13 April, however, by which time the paper has been published.]
Tuesday 13 January
Peter Pauling writes to his father, asking for a copy of his and Corey's paper on DNA structure.
Wednesday 28 January
Two advance copies of Linus Pauling and Corey's paper 'A proposed structure for the nucleic acids' arrive at the Cavendish. One is sent to Pauling's son, Peter, the other to the Cavendish's Director, Sir Lawrence Bragg. Bragg does not show Crick the paper for fear of distracting him from working on his PhD thesis. Watson grabs the paper from Peter Pauling's hand, however. The structure, a triple helix with the phosphates on the inside, is clearly wrong. This gives the Cavendish duo roughly a six-week lead in the race, as Pauling's paper will not be published until February, when the mistake will be generally noticed.
Rosalind Franklin delivers her last seminar at King's. She suggests, based on her most recent work with Ray Gosling, that the A form of DNA is not helical. (The hydrated B form and Photo 51 are not mentioned.)
Friday 30 January
Watson visits London and drops in on Franklin at King's, offering to show her the Pauling and Corey paper, but according to his account (in 'The Double Helix') he is sent away with a flea in his ear. Maurice Wilkins shows Watson an X-ray photograph taken by Franklin eight months earlier, that has recently been given to him by Ray Gosling or Rosalind Franklin (Gosling is unsure which of them passed the documents on before Franklin's departure; Wilkins says it was Gosling). It's Photo 51 of the B form. Recognising the telltale cross of a helix, Watson sketches what he can remember in the margin of a newspaper on the train back to Cambridge.
Saturday 31 January
Fearing another success from Linus Pauling (who beat Bragg to the alpha-helix of proteins) Bragg gives Watson and Crick permission to order metal components from the Cavendish workshop so that they can start model-building. (Up until this point, they had been banned by Bragg from working on DNA.)
Wednesday 4 February
The metal plates have not arrived from the Cavendish workshop, but Watson starts devising a model, based on what he can recall from Franklin's X-ray photograph. Linus Pauling writes to his son, telling him that he is having problems with his proposed 3-stranded structure.
Sunday 8 February
Wilkins comes to Cambridge for lunch with the Cricks, where he tells Watson, Crick and Peter Pauling what he knows of Franklin's work -- but it's actually very little. They try to persuade Wilkins to start making models, but he says that he'll only do it once Franklin has left King's. Crick then asks Wilkins if it's acceptable for Watson and himself to try model-making, to which Wilkins somewhat reluctantly agrees. Upset by the pressure on him, Wilkins leaves the lunch early. As he leaves, Watson comes out into the street and apologies to Wilkins.
Later that week, Cavendish colleague Max Perutz shows Watson and Crick the report of the previous December giving details of Franklin's research (Perutz serves on a committee that co-ordinates the MRC's biophysics research). Crick immediately realises that these data suggest two helices running in opposite directions (anti-parallel, as the crystal structure is similar to haemoglobin -- his own PhD subject).
[The irony here is that very similar preliminary data was probably presented at the King's colloquium by Franklin 15 months before, the one at which Watson did not take notes. If Crick had been there, he would undoubtedly have reached his conclusion in 1951. Franklin later admitted to Aaron Klug, her close colleague up until her death, that because she had never worked on single crystals or biological substances before, she had overlooked the significance of her findings for the structure of DNA for 18 months, by which time Watson and Crick had solved the problem.]
Tuesday 10 February
Franklin writes in her notebook that the B form of DNA could be made of one or two helices, but she puts it aside to work on other papers which must be finished before she leaves King's.
Thursday 19 February
Donohue explains to Watson that the chemical form of bases depicted in most textbooks is wrong (it should be 'keto', not 'enol'). This gives Watson the information he needs to work out the base-pairing mechanism.
Saturday 21 February
Pauling and Corey's note announcing their forthcoming structure paper appears in Nature.
Monday 23 February
Franklin returns to the analysis of her DNA photographs.
Tuesday 24 February
In her private notes, Franklin writes that both the A and B form of DNA must be two-chain helices, although, crucially, she does not appreciate that they must be anti-parallel nor the significance of the base-pairing.
Saturday 28 February
The Cavendish workshop has still not made the metal plates for the putative DNA model. 16-year-old Mike Fuller, a technician in the Cavendish workshop, cycles round Cambridge to buy cardboard from which Watson can make the shapes of the bases. Playing with cardboard cut-outs, Watson solves the problem of base-pairing. The shape of an A-T base pair is similar to that of a C-G pair. The rest of the structure falls neatly into place.
Watson and Crick rush off to The Eagle pub where Crick (according to Watson) tells bemused lunchtime drinkers in the 'RAF bar' that they have discovered the "secret of life". [Crick has no recollection of this episode although he does remember going home and telling his wife Odile about it. Years later she told Francis that she hadn't believed a word of it. "You were always coming home and saying things like that", she said.]
Saturday 7 March
In the evening, Crick finishes assembling the 2-metre high (10 base-pair) DNA model that he has been working on since Wednesday then goes home, exhausted, to bed.
Thursday 12 March
Watson writes to Max Delbrück in the USA, telling him about the double helix structure, and asking him not to tell Pauling about it (which, after much soul-searching, Delbrück does).
At John Kendrew's invitation, Wilkins travels to Cambridge to see the double helix model, then returns to London where he tells, in his words, "everyone" at King's about it. He rather bitterly declines Crick's offer to be listed as a co-author of the letter to Nature [a move which, years later, he says he regrets].
Tuesday 17 March
Franklin and Gosling, still unaware of the Watson-Crick model, complete the draft of their paper on the B form of DNA.
Wilkins receives a draft copy of the Watson and Crick letter to Nature.
Wednesday 18 March
Wilkins writes to Crick, requesting changes to the typescript. The phrase "very beautiful", referring to the experimental work done at King's, is deleted. Crick uses the same phrase to describe the double helix in a letter to his son. [Historian Lynne Elkin has suggested that this and other changes requested by Wilkins downplayed Franklin's role in the work. A copy of an original (unaltered) typescript can be seen in the Pauling archives.] Wilkins also asks if he can publish his (and Stokes's) experimental data simultaneously with the Watson-Crick letter. He adds that Franklin and Gosling have a paper that they would like to be included in the same issue of Nature (this comes to his attention as he is penning the letter to Crick).
Thursday 19 March
Crick writes to his son Michael, who is away at school, explaining how the "very beautiful" structure that he and Watson have devised might show how DNA replicates.
Wednesday 1 April
Bragg, still recovering from 'flu, visits the Cavendish lab show Gerard Pomerat of the Rockefeller Foundation round. They both go to see the model. Bragg authorises Watson and Crick's letter to Nature. In his diary, Pomerat describes Watson and Crick as "...somewhat mad hatters who bubble over about their new structure in characteristic Cambridge style and it is hard to realize that one of them is an American."
Thursday 2 April
A letter, typed by Watson's sister Elizabeth (Betty) and with a figure drawn by Crick's wife Odile (a former art student), is posted by Bragg, together with papers by Wilkins, Stokes and Wilson, and Franklin and Gosling to the editor of Nature. The paper, like all of those published by Nature in the 1950s, is not peer-reviewed, so publication is rapid.
Friday 6 April (approximately)
Linus Pauling, now permitted to travel, visits Cambridge and sees both the Watson-Crick model and Franklin's X-ray photo of the B form. He then goes with Bragg to the Solvay Conference on Chemistry, held in Brussels from the 8-13 April. That year the subject of the meeting is proteins. The first public account of the Watson-Crick model is given by Bragg. Singer, who provided the DNA sample to Wilkins, enabling the structure to be determined, also attends.
Saturday 25 April
Watson and Crick's letter is published in the scientific journal Nature, alongside the papers by Wilkins, Stokes and Wilson, and Franklin and Gosling.
Thursday 21 May
With the encouragement of an undergraduate friend, Antony Barrington-Brown visits the Cavendish Laboratory to photograph Watson and Crick alongside their DNA model. He receives 10/6 (52.5p) for the job from Time magazine, but the photographs are not used (some appear later in Watson's 1968 account 'The Double Helix').
Friday 29 May
Edmund Hilary and Tenzing Norgay, in an expedition sponsored by the Joint Himalayan Committee of the Alpine Club of Great Britain and the Royal Geographic Society, become the first human beings to set foot on the summit of Mount Everest, the highest point on the Earth.
Saturday 30 May
A second paper by Watson and Crick (the order of names, decided, as before, by the toss of a coin) appears in Nature. In it, they speculate about the genetical implications of the double helix structure.
Monday 1 June
James Watson boards a BOAC Constellation at Heathrow Airport bound for Cold Spring Harbor Laboratory, Long Island. Tucked under his arm is Tony Broad's newly-made demonstration model of DNA.
Tuesday 2 June
News of the conquest of Everest reaches London. As Watson's plane approaches Long Island, the captain informs passengers of the news. Back in London, Her Majesty Queen Elizabeth II is crowned at Westminster Abbey.
The Watson-Crick model makes an appearance at the Royal Society's Summer exhibition, listing Watson, Crick, Wilkins, Franklin, Stokes and Gosling as contributors.
Maurice Wilkins went on to prove that the B form of DNA could be found in any species, and spent seven years refining the precise molecular details of the structure suggested by Watson and Crick. Linus Pauling took pleasure in correcting a mistake in the number of hydrogen bonds between G and C bases proposed by Watson and Crick.
Rosalind Franklin moved to Birkbeck College and worked with Aaron Klug on the structure of viruses. She died of ovarian cancer in 1958, aged just 37 years, shortly after her model of tobacco-mosaic virus was unveiled at an exhibition in Brussels.
In 1962, Watson, Crick and Wilkins were awarded the Nobel Prize for "for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material". Nobel Prizes are not awarded posthumously. Wilkins mentioned Franklin very briefly in his Nobel Lecture (Watson and Crick spoke about their work on RNA and the genetic code respectively, and not DNA structure).
In addition to his work on the double helix, Francis Crick made several fundamental contributions to our understanding of life. With Sydney Brenner he was instrumental in helping to unravel the genetic code and the mechanisms of protein synthesis.
After the publication of 'The Double Helix' in 1968 revealed that Perutz had handed over Franklin's data (which was not, strictly-speaking, confidential), Perutz wrote to many fellow scientists seeking to be exhonerated for his action. Watson also wrote a public letter of apology to Perutz.
For a period in the 1970s it looked as though the Watson-Crick structure might be wrong, as it seemed to be incompatible with scientists' developing understanding of DNA replication. Alternative side-by-side and 'warped zipper' models gained some popularity.
In the early 1980s, however, evidence was finally provided that confirmed the double helical structure of DNA. Short identical lengths of DNA were synthesized artificially, and X-ray analysis of them gave far more accurate results than had been possible using naturally-occuring mixtures.
Francis Crick was appointed a member of the Order of Merit in 1991. He died on 28 July 2004, aged 88. Maurice Wilkins died in October 2004, nine weeks short of his 88th birthday.